Figure 1. A) Location map and B) chronology of key European Middle Pleistocene hominin samples relative to the benthic foraminiferal oxygen isotopic stratigraphy (Lisiecki and Raymo, 2005).
One of the difficulties with palaeontology is fitting individual fossils into a taxonomic system that was designed for classifying contemporaneous organisms. This difficulty is inevitable given that species change over time as they evolve so it is not a flaw in the theory but a confirmation of it, rather like the problem of deciding exactly where one colour changes into another in a rainbow is not a flaw in the theory of why sunlight can be split into different colours but a confirmation of the theory.
Where do the colours change?
Just such a problem was illustrated by a paper published recently by a team of paleoanthropologists from England and Spain, led by Lucile Crété of the Centre for Human Evolution Research (CHER), The Natural History Museum, London, UK.The team set out to decide whether hominin fossils from Boxgrove in West Sussex, England were of the same paleodeme as those found at the Sima de los Huesos (Pit of Bones) at Atapuerca, Castile and León, Spain.
What they discovered refutes Creationist assertion that there are no transitional fossils, although getting any Creationist to define a 'transitional' fossil or to describe what they expect one to look like, is like trying to get blood from a stone. In fact, of course they've no idea what one would look like, and are too afraid to even hazard a guess in case one turns up meeting their description. It is simply an article of faith that there are none, and their claims are no more substantial than the squawking of a parrot who has no idea what its noises mean.
So, I'll help them out by defining a 'transitional' fossil as part-way between an earlier form of an evolving species and a later one - which pretty much covers every fossil ever found because every fossil ever found will be from between its ancestors and its descendants.
What would we expect such a transitional, fossil to look like? Given that there is no requirement for every anatomical feature to change at a constant rate during evolutionary history, and no requirement for evolution to proceed at the same pace in all geographical locations, we would expect some features to be more like the future form and some to be more like the ancestral form. The 'transitional fossil' will be a mosaic of archaic and more modern features.
Figure 11. Wall thickness analysis in A) Boxgrove 1 (NHMUK PA EM 3566); B) recent modern humans; and (C) SH (Tib-XII). Color scale: from thickest (red) to thinnest (green) bone cortex.
By comparing the partial left tibia and two lower incisors from separate adult individuals, recovered from deposits dated to about 480,000 years ago, at Boxgrove, with a much larger sample of fossils dated to about 430,000 years ago, from Atapuerca, Spain, the team were able to show that the teeth from Boxgrove were similar enough to those at Atapuerca to place them in the same paleodeme. The Boxgrove teeth were also sufficiently like Neanderthal teeth to include them in the Homo neanderthalensis taxon, which the Atapuerca fossils have recently been assigned to, having initially been classified as H. heidelbergensis (an ancestor of Neanderthals).
But, when it came to the tibia from Boxgrove, it was not that straightforward. Although the Boxgrove tibia had the same thick cortices and bone walls as Neanderthals and the Atapuerca fossils, they had more modern features, having "a straight anterior tibial crest and a suggestion of a lateral concavity".
In other words, although the Boxgrove fossil predated the Atapuerca fossils by some 50,000 years, it had some more advanced features. The researchers concluded therefore that the Boxgrove fossils may not all have been from the same paleodeme.
In the abstract to their open access paper in the Journal of Human Evolution, they say:
AbstractThis is just about as good an example as you'll get of fossils (and populations or paleodemes) being 'transitional' between earlier and later forms and the fact that all fossils are transitional being the reason it is difficult and often controversial to place any one fossil in any one of several related taxons with any degree of certainty.
The early Middle Pleistocene human material from Boxgrove (West Sussex, UK) consists of a partial left tibia and two lower incisors from a separate adult individual. These remains derive from deposits assigned to the MIS 13 interglacial at about 480 ka and have been referred to as Homo cf. heidelbergensis. The much larger skeletal sample from the Sima de los Huesos (Atapuerca, Spain) is dated to the succeeding MIS 12, at about 430 ka. This fossil material has previously been assigned to Homo heidelbergensis but is now placed within the Neanderthal clade. Because of the scarcity of human remains from the Middle Pleistocene and their morphological variability, this study assessed whether the Boxgrove specimens fit within the morphological variability of the homogeneous Sima de los Huesos population. Based on morphometric analyses performed against 22 lower incisors from Sima de los Huesos and published material, the data from the Boxgrove incisors place them comfortably within the range of Sima de los Huesos. Both assemblages present robust incisors distinct from the overall small recent Homo sapiens incisors, and Boxgrove also aligns closely with Homo neanderthalensis and some other European Middle Pleistocene hominins. Following morphological and cross-sectional analyses of the Boxgrove tibia compared to seven adult Sima de los Huesos specimens and a set of comparative tibiae, Boxgrove is shown to be similar to Sima de los Huesos and Neanderthals in having thick cortices and bone walls, but in contrast resembles modern humans in having a straight anterior tibial crest and a suggestion of a lateral concavity. Based on the patterns observed, there is no justification for assigning the Boxgrove and Sima de los Huesos incisors to distinct paleodemes, but the tibial data show greater contrasts and suggest that all three of these samples are unlikely to represent the same paleodeme.
Lockey, Annabelle L.; Rodríguez, Laura; Martín-Francés, Laura; Arsuaga, Juan Luis; Bermúdez de Castro, José María; Crété, Lucile; Martinón-Torres, María; Parfitt, Simon; Pope, Matt; Stringer, Chris Comparing the Boxgrove and Atapuerca (Sima de los Huesos) human fossils: Do they represent distinct paleodemes?
Journal of Human Evolution (2022) 172, 103253; DOI: 10.1016/j.jhevol.2022.103253
Copyright: © 2022 The authors.
Published by ScienceDirect (Elsevier B.V). Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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